The invention relates to the field of linear SC circuit arrangements. In particular, the invention relates to linear SC circuit arrangements using integrated deep submicron technology which has at least one switched capacitor circuit, a control circuit and an output stage which is connected downstream of the switched capacitor circuit.
In the text below, an SC circuit arrangement is to be understood to mean a circuit which has at least one switched capacitor. Such switched capacitors, which are usually also referred to as “switched capacitor” circuits or SC circuits, are known generally in a multiplicity of embodiments and applications. Applications with such switched capacitors can be found, by way of example, in switched capacitor filters (see U. Tietze, Ch. Schenk, Halbleiterschaltungstechnik Seimeonductor Circuitry, 12th edition, page 866-871), SC subtractors and the like.
Switched capacitor circuits are provided in order to simulate the characteristic of a resistor in the best possible manner. This is possible only if the controllable switches or transistors have a turn-on resistance Ron which is as linear as possible. However, this can be ensured only if these controlled switches are operated in the linear range of their characteristic curve. The problem with this, however, is that reducing the actuation potential means that the corresponding transistors are simply no longer operated in ideal fashion in the linear range of their characteristic curve, a direct result of which is also a nonlinear turn-on resistance Ron.
The switched capacitances can be used to provide a filter whose filter parameter is independent of the absolute capacitance value; in particular, the cut-off frequency may in this case be set variably over a wider range. In addition, the resonant frequency, the quality factor and the gain at the resonant frequency may also be set independently of one another. In order to achieve the same functionality with conventional filters, it would be necessary to provide a universal filter, at least of second order, having filter coefficients which can be set independently of one another. However, the advantage of an SC filter over a universal filter of this type is the opportunity of simpler implementation. SC filters are used, in particular, in telecommunications, for example in transceiver circuits, broadband applications and linecard applications.
As in the case of most integrated circuits, such applications also have the increasing need to provide an ever greater level of integration and, related to this, ever smaller feature sizes for future integrated circuits. Current and future generations of large scale integrated circuits therefore use “deep submicron” technology. Deep submicron technology denotes semiconductor technologies where the corresponding integrated circuits are produced with a minimum feature size of no more than 0.25 μm, particularly no more than 0.2 μm. In semiconductor technologies used to date, where feature sizes of greater than 0.25 μm have thus been used, the supply voltage for supplying the integrated circuit was still sufficiently high to actuate the corresponding control connections of the controllable switches with a control signal which was such that the linearity demands of these switches were still met.
In integrated circuits produced using deep submicron technologies, however, a much lower supply voltage of, by way of example, less than 2 volts is used. The problem with this is now that such low supply voltages mean that the actuation potentials for actuating the control connections of the controlled switches are also not high enough to provide a sufficiently linear characteristic curve for the controlled switch. Linear switches are an absolute necessity, besides linear capacitances, for implementing linear switched capacitor circuits, however.
For this reason, the controlled switches are linearized by providing switched capacitor circuits produced using deep submicron technologies with a precharging circuit which is used to precharge the control connections of the controlled circuit. This method is also referred to as a “boot strapping” method and is described, by way of example, in U.S. Pat. No. 6,118,326, U.S. Pat. No. 5,945,872 and U.S. Pat. No. 6,060,937. Here, a voltage store is first precharged and is then clamped between the input connection of the SC circuit and the control electrode of the controlled switch which is provided for switching the switchable capacitance. This adds the control voltage precharged in the voltage store to the input signal.
The problem with this method, however, is that the signal source for the input signal is loaded to a peak during the controlled switch's changeover operation. This signal source, which is in the form of an operational amplifier or in the form of an output stage, for example, now needs to be much more powerful in order to provide the requisite, peak current for the changeover operation or to drive the controlled switches and to charge the capacitances. This equally means an increase in the power loss, which is to be avoided, however, particularly in applications with a local power supply.
The fact that the signal source briefly needs to provide a very large current means that it needs to be oversized accordingly, a direct result of which is also a larger area involvement for the integrated circuit.
Another problem is that the well substrate diodes in the integrated controllable switch, which are subjected to brief but relatively large voltage spikes during the changeover operation, experience additional loading which can result in these elements being damaged or failing. To prevent this, the well substrate diode also needs to be made more robust. This results in reliability problems for the controllable switch from time to time, however.
The document Yung, W. et al., Process Dependency of MOSFET Depletion Mode MOS Capacitors in Series Compensation, Proc. 45th Midwest Symposium on Circuits and Systems, Aug. 2002, No 1, pages I-263 to I-266, describes a switched capacitor integrator and a second-order SC/low-pass filter. This switched capacitor integrator and the second-order SC/low-pass filter can be produced in various CMOS production processes, namely in 0.35 μm, 0.25 μm and 0.15 μm (deep submicron), and can be compared with one another. However, this document gives no indication at all of the transistors in the switched capacitor circuit being in the form of thick oxide transistors.
U.S. patent specification U.S. Pat. No. 6,636,083 B1 describes a self-adjusting current source for low-power SC circuits. The self-adjusting current source is used to reduce the transistors' leakage current which arises with deep submicron processors.
The present invention is now based on the object of providing an SC circuit arrangement of the type mentioned in the introduction which permits more efficient and, in particular, more robust implementation of linear switched capacitor circuits.